1. Field of the Invention
The present invention relates to the field of current limiting circuits.
2. Prior Art
Current limiting circuits of various types are well known in the prior art. Such circuits are commonly used to limit the maximum current delivered to a circuit or through a circuit component to avoid excessive heating in the circuit or circuit component and/or an excessive load and/or overheating in the power supply itself. Of particular importance to the present invention, however, are circuits using clocked switches, such as various types of voltage converters.
In the case of switching converters using inductors, energy from a power supply is alternately stored in an inductor and then delivered to the output of the converter. In the case of the energy storage cycle, the current in the inductor will increase at a rate equal to the voltage applied across the inductor divided by the inductance of the inductor. The peak current must be limited, however, to avoid saturation of the inductor, as once the inductor saturates, the inductance drops drastically, resulting in high current spikes, excessive heating of the associated components, high loads on the power supply, etc. In the prior art, a sense resistor is commonly used in series with the inductor, with the voltage drop across the sense resistor being used to limit the current through the inductor. As shall subsequently be seen, the present invention offers an alternative to the use of the sense resistor, particularly in integrated switching converters which include the main switching transistor as part of the integrated circuit.
In the case of charge pump converters and inverters, the switch currents have different characteristics. By way of example, in the case of a voltage doubler, a capacitor is first switched across the input to the doubler to charge the capacitor to the input voltage. The capacitor is then switched so that the capacitor lead previously connected to the low side of the input is now connected to the high side of the input, so that the second lead of the capacitor providing the output of the doubler is now at twice the voltage of the doubler input.
A voltage inverter is similar in operation to a voltage doubler, though is different in the capacitor connection during the second phase of operation. A schematic representation of such a voltage inverter may be seen in FIG. 1, which is representative of such devices as the MAX828/MAX829 devices, manufactured by Maxim Integrated Products, Inc., assignee in the present invention. As with a voltage doubler, during the first phase of operation the capacitor C1 is connected between the input voltage VIN and ground by closing switches S1 and S3 to charge capacitor C1 to the input voltage. Then switches S1 and S3 are opened and switches S2 and S4 are closed, connecting the side of the capacitor that was originally connected to the input to the circuit ground, so that the opposite side of the capacitor originally connected to circuit ground will be at the voltage xe2x88x92VIN. Capacitor C2 provides smoothing and charge storage for the output voltage, with inverter I1 providing the alternate switch closings in response to a clock signal. In that regard, the alternate switch closed waveforms are non-overlapping to avoid momentary shorting of the input or the output to ground.
In the case of a charge pump voltage inverter illustrated in FIG. 1, switches S3 and/or S4 may be diodes rather than switches. Similarly, in a voltage doubler, two of the four devices may be diodes rather than switches, diodes simplifying the circuits but increasing the power dissipation and decreasing the voltage (positive or negative) obtained. In that regard, all four devices are rectifying devices in the sense that current only flows through each device in one direction, though two of the four devices must also at times be blocking devices, even though the same remain forward biased. Thus two of the four rectifying devices can be diodes, though the other two devices must have a switching capability. Of course, voltage doublers and inverters are merely exemplary of the many various circuits in which charge pumps are used, many charge pump circuits being used to obtain different voltages, both higher and lower, than merely doubling or inverting a supply voltage.
In the case of a charge pump circuit such as a voltage doubler or voltage inverter, the output of the circuit will have a finite impedance dependent upon the effective resistance of the pumping or fly capacitor, the resistance of the switches and the inverse of the product of the clock frequency and capacitance of the fly capacitor. In essence, the capacitor pumps a charge to the output on each clock cycle to provide the load current until the next clock cycle. Consequently, the fly capacitor (capacitor C1 of FIG. 1) must be sufficiently large for a given clock frequency to deliver the required charge to the output on each cycle without an excessive voltage drop on the capacitor during the second phase of operation of the circuit when the capacitor is coupled to the output. By way of example, assume a 5 volt inverter is to provide a xe2x88x925 volt output with a 100 millivolt droop in the output due to the output load (xe2x88x924.9 volts out). During start-up, however, when the output capacitor (capacitor C2 of FIG. 1) is discharged, capacitor C1 will be nearly fully discharged on each of the first few cycles of operation, resulting in switch currents which are approaching 50 times the current required for steady state operation. In the case of a fault condition wherein the output of the circuit is shorted, these extraordinary switch currents may persist indefinitely. While on start-up, switch resistances will limit the peak current and the short time required for start-up will limit the total energy dissipated in the switches and power supply, a short circuit condition for any length of time can cause excessive heating in switches, leading to possible failure of the circuit, as well as drawing excessive power from the power supply.
Current limiting using capacitor charge measurement to limit the supply current or load current of a circuit, or the current flowing through a device or switch, to prevent the time average current in the switch from exceeding a certain safe level. A replica circuit is used to provide a scaled version of the current in the main switch. The current output of the replica circuit is used to charge a capacitor during the first period of the clock signal so that the capacitor voltage, at any time during the on time, is proportional to the time integral of the current output of the replica circuit, and thus, the time integral of the current in the main circuit. The capacitor voltage, in turn, is compared with a known voltage to determine whether the charge that has flowed through the main switch has exceeded a predetermined maximum. The output of the comparator may be used in various ways, including as a control of the clock duty cycle to limit the on time of the main switch. Various embodiments are disclosed.